Surgical removal of epileptogenic brain tissue is indicated for treatment of many medically refractory focal seizure disorders, epilepsy being by far the most common. One of the important factors in providing good results from such surgery is the degree of accuracy in identifying epileptogenic foci. This involves sensing of cortical electrical activity using various types of diagnostic electrodes and, especially, the electrical contacts which form a part of such electrodes.
In recent years many epilepsy centers have used intracranial recording techniques to better define regions of cortical epileptogenicity. Broadly speaking, intracranial sensing techniques have used two different kinds of electrodes for engagement with brain tissue. These different kinds include depth electrodes which are long, thin devices inserted into the brain and having one or more electrical contacts arranged along their length. Another kind of electrode is the subdural type which is placed between the dura and the brain and in contact with the brain, but not within the brain. Such subdural electrodes are known either as strip or grid electrodes, depending primarily on whether they have one or more rows, respectively, of electrical contacts.
These kinds of electrodes each have an electrode body which is formed of a dielectric material. Depth electrodes typically have a thin, tubular body with ring-like contacts sleeved over and spaced along the body. These contacts touch brain tissue to sense electrical signals present in the tissue. A separate lead wire connect to each of the contacts and extends inside the tubular body in a direction away from the distal end of the depth electrode, i.e., that end which is inserted into the brain.
Subdural strip and grid electrodes each have at least one metallic contact and preferably a plurality of such contacts supported by the body in a spaced relationship one to the other. The electrical contacts and the lead wire extending from each are held between two thin, flat layers of dielectric material which are joined as one in the assembly process. One of these layers has a hole through it for each contact in the electrode. Such holes permit the contact to touch brain tissue and, like the contacts of the depth electrodes, directly sense electrical signals therefrom.
Depth electrodes are shown in U.S. Pat. No. 4,245,645 (Arseneault et al.) while subdural electrodes are shown in U.S. Pat. No. 4,735,208 (Wyler et al.).
Knowing the precise locations of the contacts of such electrodes is essential for accurate interpretation of the electrical signals which they sense. Electrical signals picked up by intracranial contacts can be accurately associated with a specific location in the brain only to the extent that the precise locations of the contacts vis-a-vis the brain are known. Contact location is by the use of x-ray, computerized axial tomography (CAT) and/or, more recently, magnetic resonance imaging (MRI) techniques. Since surgical removal of diseased brain is an intended subsequent course of action, accuracy in determining the location of diseased brain is of paramount importance. The substantial risks involved with removal of brain tissue are apparent.
Heretofore, such electrodes have used contacts and lead wires which are made of stainless steel or of a precious metal. With the advent of MRI techniques, on a commercial level in about 1984 in the United States, certain difficulties have been encountered in using stainless steel for such contacts and lead wires. These difficulties have persisted and until the advent of the inventive electrode, have defied solution.
Specifically, electrodes which use contacts and lead wires of stainless steel (or other biocompatible ironbearing materials) produce what is known as an artifact. An artifact is an area of very significant image blurring which extends from and beyond the edge of the electrical contact and the lead wire and prevents the treating physician from understanding the precise location of the contact. Since the identification of epileptogenic foci is extremely important in successfully treating focal seizure disorders, the presence of the artifact or image blurring makes such treatment unnecessarily difficult. While precious metals do not present such difficulties, the cost of using such metals (including platinum) is prohibitive.
Yet another difficulty relates to the fact that the electrical contacts on depth electrodes are sleeve-like and hollow and cylindrical in shape. When such contacts are made of stainless steel or other iron-bearing materials, the use of MRI equipment may cause stray magnetic flux to circulate in the contact and the result is an even more significant location-obscuring artifact.
Still another disadvantage which arises from the use of stainless steel or other iron-bearing contact and wire materials is that they exhibit magnetic properties. As a result, electrodes which employ such contacts and lead wires tend to move from the intracranial location or at least are urged toward movement. This characteristic is particularly disadvantageous in the case of subdural electrodes. Such subdural electrodes, unlike depth electrodes which are lodged in tissue, are somewhat more free to move in their positions between the dura and the brain tissue.
In addition, diagnostic electrodes of the kinds described above may also be used with x-ray diagnostic techniques. With such techniques, it is equally important for the treating physician to be able ascertain the precise locations of the contacts of such electrodes. Checks on the precise location of the contacts of such electrodes by the use of x-rays has been difficult primarily because of the nature of the electrical contacts. This is particularly the case with subdural strip and grid electrodes where the contacts are so thin and delicate that they can not be seen or seen readily at desired x-ray powers.
There is a significant and long-felt need for an improved diagnostic electrode which permits precise location of the intracranial positions of the electrical contacts when the electrodes are used with MRI diagnostic techniques. It would be additionally advantageous for such an electrode to be constructed so that the location of its contacts can be accurately determined when using x-ray diagnostic techniques.